Method and apparatus for coating by thermal evaporation



Sept. 11, 1956 F. K. TRUBY 2,762,722

METHOD AND APPARATUS FOR COATING BY THERMAL EVAPORATION Filed May 18,1953 .INVENTOR.

FRANK K. TRU BY ATTORNEY United States Patent Ofi 2,762,722 PatentedSept. 11, 1956 ice METHOD AND APPARATUS FOR COATING BY THERMALEVAPORATION Frank K. Truby, Socorro, N. Mex., assignor to Bausch &

Lomb Optical Company, Rochester, N. Y., a corporation of New YorkApplication May 18, 1953, Serial No. 355,557 14 'Claims. (Cl. 117106)This invention relates generally to evaporation coating in a vacuum andmore particularly to an improved apparatus and method for depositingcoatings onto a base.

The subject matter of this patent may be used by or for the UnitedStates Government for governmental purposes Without the payment of anyroyalties to me.

As the art of high vacuum evaporation of materials continues to develop,new requirements demand improvements over the existing devices. One bigdemand has been for some practical way to support within a vacuumchamber a relatively large supply of material so that practically all ofit can be successfully evaporated onto a base. This problem arosebecause some processes require coatings of material which are severalhundred quarter wave lengths thick while other processes require a thincoating of material to be applied to a relatively long, continuous stripor the like. Existing prior devices cannot meet this demand. Forexample, some prior devices have heating filaments disposed over thematerial being evaporated so that only a relatively small amount ofmaterial may be evaporated before the surface of the material becomestoo remote for evaporation to continue.

Another demand has been to devise some new way to evaporate materialwithout the evaporation heating filament excessively raising thetemperature of the base. For instance, some materials deposit better oncool bases, but

due to the large amount of heat radiated from the large exposedevaporation filaments, the bases are soon heated to quite a hightemperature with resulting complications.

Other materials deposit better on bases that are maintained at someconstant temperature. However, the prior art evaporation filamentscontinually radiate heat onto the bases to destroy the constanttemperature level required.

Therefore, it is an object of my invention to provide means in anevaporation apparatus for holding a relatively large supply of coatingmaterial so that substantially all of the material can be evaporatedwithout difliculty.

Another object of my invention is to provide a new and improvedapparatus for vaporizing a coating material without heating thesubstrate or base.

A further object of this invention .is to provide a new and improvedapparatus for uniformly depositing relatively thick layers of coatingmaterial on a base.

A still further object of my invention is to provide an improvedapparatus for easier and more accurate control of the deposition of acoating on a base.

Still another object of my invention is to provide an apparatus fordepositing a coating that is satisfactorily durable and wear resistantfor many purposes.

Another object of my invention is to provide a new method for preparingand evaporating a coating material onto a base.

These and other objects and advantages reside in certain novel featuresof construction, arrangement and combination of parts and in the methodsteps and method as will hereinafter be more fully explained and pointedout in the appended claims.

Referring to the drawings:

Fig. 1 is a sectional view of an evaporating unit embodying one form ofmy invention.

Fig. 2 is a top view of the new material holding device of Fig. 1

Fig. 3 is an enlarged vertical sectional view through the materialholding device after some of the material has been evaporated.

A preferred form of the present invention is shown in Fig. 1 whereinsimilar reference characters designate corresponding parts throughoutthe several views. The apparatus for coating a base according to thisinvention comprises a stand 10 on which is positioned a bell jar or thelike 11 to form a vacuum chamber 12 having a work support 13 comprisinglegs 14 supporting a metal plate 15 which has seats for holding bases orworkpieces such as lenses 16. The vacuum chamber 12 is evacuated bymeans of a vacuum pump 17 connected to the chamber by means of a conduit18.

Upstanding from the stand 10 is a pair of insulated supports 19 and 20holding, respectively, the ends of a continuous tungsten wire 23 havingan upstanding filament or heating element 24 comprising sides 25 and 26terminating in a common end 27. It is to be understood that the hairpinshape of the heating element 24 is for illustration only, and othershapes that accomplish the objects of the invention are to be includedwithin .the scope of the invention. A source of electric power isconnected to the wire 23 through a pair of lead-in wires 21 entering thevacuum chamber through the sealing plugs 22 in the stand 10. A sheet ofmica 28 or other insulating material is disposed on the supports 19 and20 and has an aperture 29 through which the heating element 24 projects.The sheet of mica 28 is disposed between the wire 23 and the bases 16 toshield the bases from the heat of the wire and to act as a support for acrucible 30;

form of my invention has compacted into the crucible 30 a suitablecoating material forminga block or mass 36 having a central, verticalcavity or opening 37 extending from an orifice 38 in the top surface ofthe mass at the open end of the cup-shaped crucible through the block ormass 36 to an orifice 39 aligned with the aperture 34 in the bottom 33of the crucible 30. The material used must be of a type that sublimesand is generally available in powder, granular or chunk crystal state.The powder or granular state of the material can be packed together toform a block or mass 36 which is self-rigid or it can be held in thecrucible 30, as shown. The opening 37 .is formed in the mass or block 36when the massis being compacted or the opening 37 can be drilled orforced in the block or mass after compacting. The chunk crystal form ofthe material is in block or mass state and must have an opening orcavity 37 drilled or formed into the body of the mass. When the block ormass 36 is made of chunk crystal, .it is self-rigid and need not beplaced in the crucible 30, but it may be so disposed if desired.

The crucible 30 and its block or mass of coating material 36 is adaptedto fit around the heating element 24 and to rest on the crucible bottom33 upon the sheet of mica 28. For most materials the heating element orfilament 24 extends up through the aperture 34 in the crucible and intothe opening 37 in the compacted mass toa point just below the orifice 38in the top surface thereof. However, some materials, i. e., cryolite,when heated will slowly grow over the orifice 38 of the open ing 37 ofthe material if the end 27 of the filament 24- is below the orifice.With this type material, the heat ing element 24 is extended slightlybeyond the top surface of the block or mass of material to keep theorifice open. When the block or mass 36 is formed either of theselfrigid crystal type or of the self-rigid compacted type of material,it can be placed directly on the sheet of mica 28 with the filament 24extending into the opening 37. The opening 37 in the block or mass is ofsuch a size and shape that the heating element 24 is substantiallycoextensive with and spaced from the wall portions of said opening. Inthis way, the coating material will not be contaminated by contact withthe filament or heating element 24.

The coating material ,36 must be of the type that sublimes, goingdirectly from the solid to the vapor state when heated. Materials suchas zinc sulfide, cryolite, chiolite and the like fall generally in thegroup that sublime. Since the materials sublimate, when the filament orheating element stays below the orifice of the open ing in the material,the mass 36 will be disintegrated by radiant heat in the region aroundthe heating element 24, leaving intact the material around the orifice38 in the end of the opening 37 until the shape of the block or massassumes the appearance of Fig. 3 in vertical section. The orifice 38remains substantially fixed in shape and diameter due to the end 27 ofheating element 24 being disposed below the exposed surface of the blockor mass so that not enough heat is available to sublimate the materialaround the orifice 38. This orifice 38 will act as an apertured mask orshield which always permits the same crosssectional cone or pattern ofevaporated material to be projected from the block or mass along thepath toward the workpieces .or bases 16. After a certain amount of use,the original orifice 38 will disintegrate somewhat whereby the orificewill be enlarged. If desired, an apertured mask may be placed on top ofthe crucible so that the aperture in the mask will permit the materialto escape in a preselected cone or pattern shape.

When the filament or heating element 24 extends beyond the orifice 38 ofthe opening 37 in the material, the orifice may not develop as shown inFig. 3, .but the evaporation will take place coextensive with thefilament 24. While under this last-mentioned condition, a certain amountof control over the size of the cone of emitted material will belacking, it will still be effective for evaporating large quantities ofmaterial.

In many prior devices, the relatively large evaporation filaments orheating elements are exposed to the base or workpiece 16 whereby thebase or workpiece is continually heated'without any practical means ofcontrol. [he present invention is designed to overcome this heating byhaving only the small end area 27 of the filament or heating element 24exposed to the base or workpiece 16 so that even after long periods ofdepositing material many wave lengths thick, the base or workpiece 16'will not beexcessively heated by the filament or heating element 24.This removes a variable that has always been very difficult to control,especially when continuously depositing many layers of films on a baseover a period of many hours, during which time the evaporation filamentor heating element would continue to heat the base to a higher andhigher temperature. With the large heating element no longer exposed tothe bases, it is now possible with my invention to substantiallymaintain the bases at any desired predetermined temperature. Forinstance, if a base is inserted into the vacuum chamber at roomtemperature it will, by the use of my invention, remain substantially atroom temperature and if a base is heated to a preselected temperature byheaters within the vacuum system, the temperature of the base will notbe materially altered by :the evaporation filament or heating element.

When carrying out the method of my invention, the bases or lenses 16 tobe coated are placed in the evacuating chamber on the support 13 withinthe range of the emitted coating material. In one form of my invention,

a block or mass of coating material which will sublime is packed into acrucible 30 so that it has the opening 37 running therethrough from anaperture 34 in the bottom of the crucible to an orifice 38 in the topsurface of the mass. The crucible is placed in the evacuating chamberwith the orifice 38 directed toward the bases 16 and with the heatingelement 24 within the opening 37 in the block or mass so that withcertain materials the end 27 of the element is located just below theorifice 38 of the opening. After the chamber is evacuated, the radiantheat from the heating element sublimates the block or mass of materialaround the wall portions of the opening whereby the vapor of thesublimed material will be emitted from the orifice of the opening in theblock or mass along a cone path to the base to be coated.

The crucible 30 of the present invention can be made large enough tocontain a block or mass of material for depositing a coating severalhundred quarter wave lengths thick. However, even if the mass ofmaterial is of the same volume as the supply of material used in priorart devices, many times more material can be successfully evaporatedfrom the block or mass using the present invention. This is primarilybecause such prior art devices have the heating coil located so that thematerial is evaporated only from the exposed side of the material whichis adjacent to the heating coil. After a time, the material close to thecoil is evaporated away leaving the rest of the material too far fromthe heating coil to be evaporated. With the present invention, theheating element is disposed within an opening in the mass of material sothat material is evaporated from substantially the entire wall portionof the opening which is adjacent to and substantially coextensive withthe heating element. This means that a larger surface area of materialis exposed to the heating element in the present device so that muchmore material is evaporated. Since the heating coil is almost completelysurrounded by material, it has been found that the heat within theopening is not dissipated so rapidly. Therefore, the heat remains longerwithin the opening to evaporate more material from around the wallsthereof. In this way, the block will be evaporated away to a greaterdistance from the heating element than would normally be possible.

The coatings that result from using my invention have been found to bemore uniform in quality and thickness. This is believed to be due to thefact that the cone of evaporated material emitted from the orifice ofthe opening is homogeneously filled with particles of the coatingmaterial. Also, the spread of the cone is somewhat controlled so thatonly that material directed toward the lenses or bases is emitted fromthe block whereby such emitted material is practically all depositedwith very little waste.

While the block or mass of coating material 36 has beendisclosed ashaving an opening 37 extending therethrough, it is to be understood thatthe opening may not necessarily extend through the mass but may be inthe form of a pocket or cavity having a bottom. Although the heat fromelement 24 is electrically produced, it is to be understood that otherforms of heating elements may be used within the opening in the mass ofmaterial.

From the foregoing, it will be apparent that I am able to attain theobjects of my invention and provide an improved apparatus and method fordepositing coatings of higher quality onto a base. The small exposed endof the heating element will not excessively heat the bases or workpiecesas in prior devices. The coatings will be deposited more evenly sincethe cone of material emitted from the orifice of the block or mass ishomogeneous. Further, the present apparatus holds material sufficient todeposit coatings many times thicker than in prior devices. And, finally,the present crucible and block or mass of material has the advantage ofnever warping or tilting from its preset position which is a distinctadvantage over former devices, in that the material will always bedirected toward the bases.

Various modifications may obviously be made without departing from thespirit of my invention as pointed out in the appended claims.

I claim:

1. A method of evaporating a material which will sublime which comprisesforming in a mass of the material an opening having an internal wall andan orifice and vaporizing some of the material of the wall by heatlngprimarily the internal wall of said opening whereby the vapor of thematerial will be emitted from the orifice of the opening.

2. A method of evaporating a material which will sublime which comprisesforming in a mass of the material an opening having an internal wall andan orifice, inserting heating means within said opening in the mass tosubstantially surround the means with the material, and activating saidheating means to sublimate only the material forming the internal wallof the opening in the mass whereby the vapor of the sublimed materialwill be emitted from the orifice of the opening in the mass.

3. A method of evaporating a material which will sublime comprisingcompacting the material in a crucible, forming in the material anopening having an internal wall and an orifice, inserting a heatingelement into the opening to substantially surround the element with thematerial and heating the element to vaporize only the material from theinternal wall of the opening whereby the vapor will be emitted from theorifice of the opening.

4. In a method of depositing a coating material onto a base, the stepsof compacting material which will sublime into a crucible, forming anopening through the material in alignment with an aperture in the bottomportion of the crucible, inserting a heating element through theaperture and into the opening to a point below the top surface of thematerial and heating the element to vaporize the material from the wallportions of the opening.

5. An apparatus for evaporation of coating materials which will sublimecomprising a crucible having an aperture therein and being adapted tohold a mass of coating material, said mass having an openingtherethrough aligned at one end with said aperture in the crucible, andheating means extending through said aperture and into the opening andbeing substantially coextensive with said opening and spaced from thewall thereof for subliming the coating material forming the wallportions of the opening.

6. Apparatus for evaporation coating of bases in a vacuum comprising achamber adapted to be evacuated, means for evaporating coating materialwhich will sublime in the chamber, said means comprising a cupshapedcrucible having an aperture in the bottom thereof said crucible beingadapted to hold a mass of material having a vertical opening extendingtherethrough and in alignment with said aperture, and an electricalheating element for subliming said material extending through theaperture and into the opening to a point below the top surface of thematerial whereby coating material from the wall of the opening issublimed and is directed through the orifice in the top surface of thematerial onto the bases to be coated.

7. Apparatus for evaporation coating of a base in a vacuum comprising achamber adapted to be evacuated, means within said chamber forevaporating a material which will sublime comprising a crucible adaptedto hold a mass of material formed with an opening having an internalwall and an orifice in one side thereof and heating means located withinsaid opening and spaced from the internal wall thereof for heating thematerial to produce sublimation, said heating means being substantiallysurrounded by the material whereby the vapor of the material will beemitted from the orifice of the opening.

8. In an evaporation apparatus having a chamber adapted to be evacuated,a support in said chamber for positioning a base within the range of anevaporated coatsurrounding a heating element with said ing material thecombination with means for evaporating the coating material comprising acrucible adapted to hold a mass of coating material which will sublime,the bottom of said crucible having an aperture therein, said mass havingan opening therethrough aligned at one end with said aperture, andheating means projecting through the aperture and into the opening inthe mass to a point below a top surface thereof and spaced from the wallportions thereof to sublime material from said wall portions around saidheating means whereby said sublimed material will be projected throughan orifice of the opening in the top surface of the mass into depositingcontact with the base.

9. An apparatus for the evaporation of materials which will sublimecomprising means for holding a mass of coating material which isprovided with an elongated opening having an internal wall and anorifice, means for holding a base to be coated in alignment with theorifice, and elongated heating means positioned within the opening andsubstantially coextensive therewith, said heating means being spacedfrom the wall of the opening and having a relatively small end portionthereof located substantially at the orifice whereby the materialforming the wall of the opening will be sublimed and directed throughthe orifice and onto the base which receives direct heat radiation onlyfrom the exposed end portion of the heating means.

10. A method of depositing a material on a base by evaporating amaterial which will sublime which comprises forming in a mass of thematerial an elongated wall and an orifice, heating only the materialformmg the wall of the opening sufiiciently to produce sublimationthereof while substantially preventing sublimation of the material atthe orifice and thereby retaining the original size of the orificethrough which the vapor of the sublimated material passes to the base.

11. A method of depositing a sublimable material on a base located abovethe material by subjecting the latter to heat from an element whilesubstantially preventing the passage of heat from the element to thebase comprising the steps of substantially surrounding the element witha mass of the material by placing the element within an opening formedin the material and having an orifice and energizing the element tosublimate the material forming the wall of the opening so that the vaporformed will pass out through the orifice of the opening and be depositedon the base.

12. A method of depositing on a base a material which will sublimecomprising the steps of forming an opening having an orifice in a massof the material, substantially material by placing the element withinthe opening, and energizing the heating element to sublimate thematerial forming the wall of the opening whereby the vapor formed willpass out of the orifice of the opening and onto a base positioned abovethe orifice and the amount of heat passing from the element to the basewill be greatly re duced.

13. A method of depositing a material on a base by evaporating amaterial which will sublime which comprises forming in a mass of thematerial an opening having an internal wall and an orifice, heating thematerial forming the internal wall of the opening to produce sublimationthereof Without heating the material at the orifice enough to causesublimation thereof whereby the vapor of the sublimated material willpass to the base through the orifice which substantially retains itsoriginal size.

14. In an apparatus of the type described having a chamber adapted to beevacuated and a support in the chamber for holding a base to be coated,the combination of means in the chamber for evaporating a coatingmaterial comprising a mass of said material which will sublime, saidmass of material having an opening extending therethrough andterminating at its upper end in an orifice which is located below thebase to be coated, a heating element extending into the lower end of theopening and terminating substantially at the orifice whereby primarilythe wall of the opening adjacent to the element may be heated tosublimation so that the vapor will pass out through the orifice and bedeposited on the base.

References Cited in the file of this patent UNITED STATES PATENTS

1. A METHOD OF EVAPORATING A MATERIAL WHICH WILL SUBLIME WHICH COMPRISESFORMING IN A MASS OF THE MATERIAL AN OPENING HAVING AN INTERNAL WALL ANDAN ORIFICE AND VAPORIZING SOME OF THE MATERIAL OF THE WALL BY HEATINGPRIMARILY THE INTERNAL WALL OF SAID OPENING WHEREBY THE VAPOR OF THEMATERIAL WILL BE EMITTED FROM THE ORIFICE OF THE OPENING.